1. Field of the Invention
The present invention concerns a radio navigation system.
2. Description of the Prior Art
At present there are two known types of radio navigation systems based on time-referenced measurements: the so-called global systems (for example the Global Positioning System or GPS) and the so-called localized systems (for example the Trident system).
The GPS system works with a receiver borne by the moving body (land-based moving body or aircraft) to be localized and transmitters borne by several satellites. The position of the moving body is identified by cross-checking several measurements of distance from a certain number (generally 3 or 4) of these satellites, each distance measurement being made by measuring the time between the transmission and the reception of one and the same signal, using one and the same time reference (for example synchronized clocks). Each satellite, in its message, provides models relative to its trajectory (ephemeris) and to the behaviour of its clock. In fact, the rough distance measured, or pseudo-distance, is vitiated by various errors such as those due to fluctuations in propagation or to the drift of the clocks. However, the former error can be reduced very substantially by comparison with a well-defined, geodesically fixed receiver (differential measurement).
The main drawback of the GPS system, however, lies in the fact that, given the mobility of the element to be localized, and the relatively long periods of time taken for propagation, acquisition, computation, etc. used in this system, it does not give the real position of the moving body in real time, but a predicted position on the basis of positions determined previously. This drawback becomes all the more appreciable as the moving body moves quickly.
The Trident system, on the contrary, using geodesic points on the ground, fitted with responder beacons, enables the real-time, high-precision localization of a moving body fitted out with an interrogator. The position of the moving body is also identified by the cross-checking of several measurements of distance from several beacons, each distance measurement being performed by measuring the time between interrogations and responses.
By contrast, the Trident system has the drawback of requiring a large number of beacons (generally about 30) on the ground and not having wide coverage due to the fact that the responder beacons are located on the ground and due to the order of magnitude of the radio-electrical wavelengths used.